1. Field of the Invention
The present invention generally relates to floating offshore platforms. More particularly, it relates to semi-submersible production platforms.
2. Description of the Related Art Including Information Disclosed Under 37 CFR 1.97 and 1.98
This invention relates to semi-submersible vessels used for hydrocarbon production (“production semi”). On semi-submersibles used for drilling operations, fluid storage within the hull is commonplace. However, the fluid quantities needed for drilling operations are typically much less than what would be required for hydrocarbon product storage on a production semi.
Most natural gas extracted from the Earth contains, to varying degrees, low molecular weight hydrocarbon compounds; examples include methane (CH4), ethane (C2H6), propane (C3H8) and butane (C4H10). The natural gas extracted from coal reservoirs and mines (coalbed methane) is the primary exception, being essentially a mix of mostly methane and about 10 percent carbon dioxide (CO2).
Natural-gas condensate is a low-density mixture of hydrocarbon liquids that are present as gaseous components in the raw natural gas produced from many natural gas fields. It condenses out of the raw gas if the temperature is reduced to below the hydrocarbon dew point temperature of the raw gas.
The natural gas condensate is also referred to as simply condensate, or gas condensate, or sometimes natural gasoline because it contains hydrocarbons within the gasoline boiling range. Raw natural gas may come from any one of three types of gas wells.
Raw natural gas that comes from crude oil wells is called “associated gas.” This gas can exist separate from the crude oil in the underground formation, or dissolved in the crude oil. Condensate produced from oil wells is often referred to as “lease condensate.”
“Dry gas wells” typically produce only raw natural gas that does not contain any hydrocarbon liquids. Such gas is called non-associated gas. Condensate from dry gas is extracted at gas processing plants and, hence, is often referred to as plant condensate.
“Condensate wells” produce raw natural gas along with natural gas liquids. Such gas is also called associated gas and often referred to as wet gas.
Raw natural gas typically consists primarily of methane (CH4), the shortest and lightest hydrocarbon molecule. It also contains varying amounts of:
Heavier gaseous hydrocarbons: ethane (C2H6), propane (C3H8), normal butane (n-C4H10), isobutane (i-C4H10), pentanes and even higher molecular weight hydrocarbons. When processed and purified into finished by-products, all of these are collectively referred to as Natural Gas Liquids or NGL.
Most large, modern gas processing plants recover natural gas liquids (NGL) using a cryogenic low temperature distillation process involving expansion of the gas through a turbo-expander followed by distillation in a demethanizing fractionating column. Some gas processing plants use a lean oil absorption process rather than the cryogenic turbo-expander process.
U.S. Pat. No. 7,980,190 entitled “Deep draft semi-submersible LNG floating production, storage and offloading vessel” describes a method, apparatus, and system of a deep-draft semi-submersible hydrocarbon, such as for liquefied natural gas (LNG), floating production and storage vessel that can include a pontoon containing hydrocarbon tanks, fixed ballast at the bottom in a double-bottom portion, and segregated ballasted tanks with variable ballast located generally above the fixed ballast portion that can assist in keeping the pontoon submerged during various storage levels. Multiple vertical columnar supports can penetrate the pontoon from top to bottom and extend above the water surface to support a deck, including various topside structures. An intermediate double-deck on the top of the pontoon can provide access to the tanks, for example, through the vertical columnar supports. The double bottom structure, deck, and vertical columnar supports are said to provide overall structural integrity.
U.S. Pub. No. 2009/0293506 A1 entitled “Semi-Submersible Offshore Structure Having Storage Tanks for Liquefied Gas” describes a semi-submersible offshore structure having storage tanks for liquefied gas, which is constructed so as to improve workability in marine offloading of the liquefied gas stored in the storage tanks while reducing an influence of sloshing. The offshore structure is anchored at sea and has liquefied gas. The offshore structure includes a storage tank storing liquefied gas, a plurality of columns partially submerged under the sea level and each having the storage tank therein, and an upper deck located on the plurality of columns to connect the columns to each other.